The invention relates to an electric fuse, in which the current interruption in the case of, for example, an overcurrent caused by a short-circuit is effected by a fusible element. Such fuses are also used, for example, to protect the high-current supply of the on-board network of motor vehicles. The input connection of the fuse is connected to the positive pole of the vehicle battery, while the output connection is connected to the on-board network.
A problem generally associated with safety fuses is their trip behavior. The length of time that passes until the fuse is tripped or the fusible element melts through is a function of, among other things, the magnitude of the overcurrent. The larger the overcurrent, the shorter the trip time. The trip time also depends on the size of the fuse or its fuse value. With the same overcurrent, a 70 A fuse, for example, trips faster than a 100 A fuse. A shortening of the trip timexe2x80x94which is not only desired for vehicle electrical systemsxe2x80x94through the use of weaker fuses is, however, impossible due to the associated danger of faulty tripping in conventional fuses.
A further problem is that, due to high-resistance line connections, for example, or a defective or inadequately-charged battery, the current flowing across the fuse does not suffice to melt the fusible element, or the current is too low to effect the melt-through in a sufficiently short time. The electrical system is consequently damaged.
Based on these conditions, it is the object of the invention to provide an electric fuse that exhibits an improved trip behavior.
This object generally is accomplished according to the invention by a fuse arrangement wherein two parallel current paths, namely a main current path and a partial current path, which connect the input and output connections of the fuse to one another, and a single fusible element and a switch are provided. The single fusible element is disposed in the main current path, and the switch is disposed in the partial current path. The switch is designed to open when predetermined limit values of the current flowing through the fuse, and/or the temperature of the fusible element, are or is exceeded. Whereas, in conventional fuses having a fusible element, the entire current flows off across the fusible element, according to the invention, the current is divided. The current flowing by way of the main current path or the fusible element is reduced by the amount of current flowing by way of the partial current path. Thus, a fuse having a lower fuse value can be inserted into the main current path. If, for example for protecting a consumer or an on-board network, a 100 A fuse is required, an 80 A fuse can be used if the partial current path is designed such that 20% of the total current flows off via the partial current path.
With the same current value, the trip time in a weaker fuse is less than in a stronger fuse. The switch of the partial current path is designed to open in the event of an overcurrent caused by, for example, a short-circuit in the electrical system. When the switch is opened, the entire overcurrent flows off via the fusible element. Because, however, the embodiment according to the invention allows the fusible element to have a lower fuse value than is normally required, the trip time is shortened relative to the strong fuse that is otherwise used. Finally, a fuse according to the invention attains the same effect as a fuse having a lower fuse value than is necessary; in this case, however, the risk of faulty trips is eliminated.
The switch in the partial current path can essentially be embodied to open when either a predetermined current value, or a predetermined maximum temperature of the fusible element, is exceeded. Both parametersxe2x80x94temperature and current valuexe2x80x94can, however, also be used simultaneously as criteria for the opening of the switch. Bimetal switches, semiconductor switches or switching elements referred to as a xe2x80x9cpolyswitch,xe2x80x9d whose resistance increases sharply when the switch is heated, are examples of suitable switches.
The safety fuse and the switch disposed in the partial current path are arranged sandwich-style, particularly if the opening criterion for the switch is the temperature of the fusible element, with the switch and the fusible element resting against one another with two contact surfaces and being in thermal contact.
Generally, the elements that are preferably used are those that automatically re-close after the fusible element has melted through or cooled. Examples of such switches include the aforementioned switches, e.g., bimetal switches, semiconductors and polyswitch elements. In a motor vehicle, the advantage is that, if the current supply of the on-board network has failed due to a short-circuit, a layman is typically incapable of replacing the defective fuse with a new one. This is because the high-load fuses of a motor vehicle are usually only accessible to auto mechanics. The failure of the on-board network takes critical vehicle functions, such as the hazard-light system or the like, out of operation. If, however, the switch re-closes after the fuse has melted through, the on-board network is supplied with current after the short-circuit has been remedied. The current flowing across the switch is reduced due to the increased resistance of the partial current path. The current is usually sufficient, however, to restore devices such as the hazard-light system or an on-board telephone to operation.
If a sustained short-circuit occurs in the on-board network, a thermosensitive switch will re-open after a certain period of time when an excessive current flows by way of the partial current path. If, however, the source of the short-circuit is remote, an uninterrupted supply is available to the on-board network, which is only the case in conventional fuse systems after the fusible element has been replaced.
Special advantages are attained with the use of a microprocessor. The microprocessor can control the switch, for example. The switch therefore need not be thermosensitive. The temperature of the fusible element can be detected by a thermosensor and reported to the microprocessor, which actuates the switch if a temperature limit value is exceeded. It is also conceivable for the fuse to include a current-measurement device, which transmits the value of the present total current flowing through the fuse to the microprocessor, which actuates the switch if a limit current value is exceeded. A further advantage of the use of a microprocessor is that it can be connected to the testing and monitoring system of a vehicle. Thus, it is conceivable, for example, that the switch is also opened simultaneously if an airbag is triggered in order to attain the earliest-possible melt-through of the fusible element in the event of a short-circuit in the on-board network. Finally, a microprocessor can be used, for example, to detect a temperature increase in the region of the input or output connection of the fuse, with the aid of a thermosensor. If the aforementioned connections exhibit an increased temperature, for example due to corrosion because of an excessive resistance, this can be reported to the driver by way of a display in the dashboard, so he is forewarned and can begin looking for an auto mechanic shop.